Resonance-Enhanced Multiphoton Ionization in the X-Ray Regime

Area of Science:

• Atomic Physics
• X-ray Science
• Quantum Mechanics

Background:

• Nonlinear ionization of atoms with intense ultrashort pulses is crucial for understanding light-matter interactions.
• High charge states in argon require specific ionization mechanisms beyond direct photoionization.

Purpose of the Study:

• To investigate the nonlinear ionization of argon atoms using ultraintense X-rays.
• To elucidate the mechanism responsible for producing high charge states, specifically Ar^{17+}.

Main Methods:

• Utilizing ultraintense X-rays from the European XFEL for nonlinear ionization experiments.
• Employing state-of-the-art theoretical calculations to model the ionization processes.
• Analyzing ion yields and spectral profiles to understand the underlying physics.

Main Results:

• Observed sequential multiphoton ionization leading to high charge states in argon.
• Identified a two-color X-ray resonance-enhanced multiphoton ionization (REMPI) mechanism involving a 1s→2p transition.
• Demonstrated this REMPI mechanism is responsible for ionization to Ar^{17+} and other high charge states.

Conclusions:

• X-ray REMPI is a distinct mechanism from conventional REMPI due to competing ionization and decay processes.
• The proposed two-color X-ray REMPI mechanism accurately explains the observed high charge states in argon.
• This study advances the understanding of nonlinear atomic ionization in the short wavelength regime.